1. Field of the Invention
The present invention relates to decoding apparatus and, more particularly, to a Viterbi decoding apparatus for use in some suitable reception unit of a CDMA (code division multiple access) system cellular telephone which is one of the mobile radio communication systems.
2. Description of the Related Art
A method of decoding a convolutional code signal by using a so-called Viteri algorithm has heretofore been proposed.
Specifically, in this decoding method, a branch metric of one code block is calculated from a received symbol, and a new remaining path is selected on the basis of the branch metric and path metric of the remaining path of each state in the preceding decoding step. Further, the path metric corresponding to the new remaining path of each state and a corresponding coder input data sequence are memorized in a path memory.
On the other hand, a most likelihood remaining path at that timing point is selected from remaining paths of respective states. Then, a coder input bit obtained at a timing point returned by an amount of path memory length along the selected path of the memorized coder input data sequence is output as decoded data. The decoding is carried out by using such decoding algorithm.
In communication systems to which the CDMA system used in mobile radio communication is applied, transmission data is coded into a convolutional code and then transmitted on the transmission side. Recently, as such communication system, there is proposed a communication system which includes a sync (synchronizing) channel (first transmission channel) having a fixed data rate in which data is continuously transmitted without a tail bit at the frame unit and a traffic channel (second transmission channel) having a data rate variable at the frame unit in which a tail bit is added to data at the frame unit and data is transmitted at the frame unit.
In this communication system, the fixed data rate of the sync channel is determined previously so that the reception side can know the data rate. On the other hand, there is proposed a communication system having a traffic channel in which data is transmitted with unknown data rate. According to this communication system, the reception side must determine a decoding data rate by discriminating data rates of unknown frames of the traffic channel.
However, there is not proposed a method suitable for determining a decoding data rate.
Further, even when data rate of every frame of the traffic channel is transmitted, such data is not always correct. Specifically, it is considered that an error occurs in transmission or decoding, for example, so that data is decoded at an erroneous data rate. Therefore, also in this case, it is requested that a data rate of every frame of the traffic channel is discriminated on the reception side.